In recent years, the improvement in the fuel efficiency of automobiles has been an important subject from the viewpoint of global environmental conservation. Therefore, by employing a high strength automobile material, there has been an active move to reduce the thickness of components and thus to lighten the automobile body itself. However, since an increase in the strength of steel sheets reduces workability, the development of materials having both high strength and good workability has been demanded. To satisfy such a demand, various multiple-phase steel sheets such as a ferrite-martensite dual-phase steel (DP steel) and a TRIP steel that uses transformation-induced plasticity of retained austenite have been developed.
Furthermore, in recent years, a high strength steel sheet having a tensile strength of more than 1400 MPa has been considered to be utilized and the development has been in progress.
For example, JP 2528387 discloses an ultra-high strength cold-rolled steel sheet having a tensile strength of more than 1500 MPa that has good formability and sheet shape by performing annealing under certain conditions, performing rapid cooling to room temperature with spray water, and performing overaging treatment. JP 8-26401 discloses an ultra-high strength cold-rolled steel sheet having a tensile strength of more than 1500 MPa that has good workability and impact properties by performing annealing under certain conditions, performing rapid cooling to room temperature with spray water, and performing overaging treatment. JP 2826058 discloses a high strength thin steel sheet that has a tensile strength of 980 MPa or higher and whose hydrogen embrittlement is prevented by forming a steel microstructure including 70% or more of martensite on a volume basis and limiting the number of Fe—C precipitates each having a certain size or larger.
However, the above disclosures pose the problems below.
In JP 2528387 and JP 8-26401, ductility and bendability are considered, but stretch-flangeability is not considered. Furthermore, there is another problem in that since a steel sheet needs to be rapidly cooled to room temperature with spray water after annealing, manufacturing cannot be performed without a line having special equipment that can rapidly cool a steel sheet and that is installed between an annealing furnace and an overaging furnace. In JP 2826058, only the hydrogen embrittlement of a steel sheet is improved. Except for a slight consideration for bendability, workability is not sufficiently considered.
In general, to increase the strength of a steel sheet, the ratio of a hard phase to the entire microstructure needs to be increased. In particular when a tensile strength of more than 1400 MPa is achieved, the ratio of a hard phase needs to be increased considerably. Therefore, the workability of a steel sheet is dominated by the workability of a hard phase. In other words, when the ratio of a hard phase is low, minimum workability is ensured due to the deformation of ferrite even if the workability of the hard phase is insufficient. However, when the ratio of a hard phase is high, the deformability itself of the hard phase directly affects the formability of a steel sheet because the deformation of ferrite is not expected. Thus, in the case where the workability of a hard phase is not sufficient, the formability of a steel sheet is considerably degraded.
Therefore, in the case of a cold-rolled steel sheet, as described above, martensite is, for example, formed by performing water quenching in a continuous annealing furnace that can perform water quenching, and the martensite is then tempered through reheating, whereby the workability of the hard phase is improved.
However, in the case where a furnace has no ability to temper the thus-formed martensite through reheating, the strength can be ensured, but it is difficult to ensure the workability of the hard phase such as martensite.
By using bainite and pearlite as a hard phase other than martensite, the workability of a hard phase is ensured and the stretch-flangeability of a cold-rolled steel sheet is improved. However, bainite and pearlite do not necessarily provide satisfactory workability and sometimes cause a problem about the stability of characteristics such as strength.
In particular when bainite is used, there is a problem in that ductility and stretch-flangeability significantly vary due to the variation in the formation temperature of bainite and the holding time.
Furthermore, to ensure ductility and stretch-flangeability, a mixed microstructure of martensite and bainite is considered.
However, to employ a mixed microstructure composed of various phases as a hard phase and precisely control the fraction, the heat treatment conditions need to be strictly controlled, which poses a problem of manufacturing stability. It could therefore be helpful to provide an ultra-high strength steel sheet having a tensile strength of 1400 MPa or higher that can achieve both high strength and good formability and an advantageous method for manufacturing the steel sheet.